Each gold medal is made up of 92.5 percent silver and 1.34 percent gold, with the remainder copper. TheInternational Olympic Committee stipulates that each gold medal must have six grams of gold (as well as 92.5 percent silver).
If the London 2012 gold medal was made of solid gold, it would cost nearly $40 million to make, according to the WGC. This is why the last time pure gold medals were presented was in 1912.
See the following for more on Olympic gold medals